Gas turbine engine geared compressor with first and second input rotors

ABSTRACT

A booster assembly for a gas turbine engine having a first rotor assembly comprising a low pressure turbine drivingly connected to a fan via a first shaft and a second rotor assembly comprising a second turbine drivingly connected to a high pressure compressor via a second shaft. The first and second rotor assemblies are arranged to undergo relative rotation in use about a common axis. The booster assembly comprises a further compressor arranged to be disposed about said common axis between the fan and high-pressure compressor in a direction of flow and a gearing having first and second input rotors and an output rotor. The first input rotor is arranged to be driven by the first rotor assembly and the second input rotor is arranged to be driven by the second rotor assembly such that the output rotor drives the further compressor in dependence upon the difference in rotational speed between the first and second rotor assemblies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/060,094, filed on Oct. 22, 2013, which is basedupon and claims the benefit of priority from British Patent ApplicationNumber GB1219544.2 filed 31 Oct. 2012. The entire contents of each ofthe above applications are incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention concerns compressors for gas turbine engines.

2. Description of the Related Art

With reference to FIG. 1, a conventional ducted fan gas turbine enginegenerally indicated at 10 has a principal and rotational axis 11. Theengine 10 comprises, in axial flow series, an air intake 12, apropulsive fan 13, an intermediate pressure compressor 14, ahigh-pressure compressor 15, combustion equipment 16, a high-pressureturbine 17, and intermediate pressure turbine 18, a low-pressure turbine19 and a core engine exhaust nozzle 20. A nacelle 21 generally surroundsthe engine 10 and defines the intake 12, a bypass duct 22 and a bypassexhaust nozzle 23.

The gas turbine engine 10 works in a conventional manner so that airentering the intake 12 is accelerated by the fan 13 to produce two airflows: a first air flow into the intermediate pressure compressor 14 anda second air flow which passes through a bypass duct 22 to providepropulsive thrust. The intermediate pressure compressor 14 compressesthe air flow directed into it before delivering that air to the highpressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 17, 18, 19 before being exhausted through thenozzle 20 to provide additional propulsive thrust. The high,intermediate and low-pressure turbines 17, 18, 19 respectively drive thehigh and intermediate pressure compressors 15, 14 and the fan 13 bysuitable interconnecting shafts.

Alternative gas turbine engine arrangements may comprise a two, asopposed to three, shaft arrangement. In one known two-shaftconfiguration, the low-pressure turbine drives the fan only and thedesired compression ratio into the combustor is achieved by amulti-stage high-pressure compressor. However it is generally desirableto increase the speed of the high-pressure compressor as far as possiblein order to increase efficiency. The maximum speed that can be achievedby the high-pressure compressor is limited by the compressor blade tipdiameter of the forward stages of the compressor, which have largerdiameter than the smaller rearward stages. Furthermore the axial loadson the high-pressure spool for such a configuration are large.

Accordingly it is generally known that a so-called booster may providedin a two-shaft engine configuration in order to provide furthercompression of the core airflow in between the fan and the high-pressurecompressor. The booster may driven by the low pressure turbine/shaft andthus rotates at the same speed as the fan. Such a booster has limitedefficiency and offers only a limited compression ratio, requiring alarge number of booster compressor stages, thereby carrying relativelylarge penalties in terms of cost, weight, engine length, and aerodynamicdrag. The axial load relief for the high-pressure spool is also limited.

As a solution to the above deficiencies it is known to provide a boosterdriven directly by the low-pressure turbine but to provide a reductiongearbox between the booster and the fan. This allows the booster and fanto be driven at optimal respective speeds/efficiencies whilst alsoreducing the axial loading on the high-pressure spool.

However it will be appreciated by the skilled person that theaerodynamic efficiency of the compressors and turbines themselves isonly one aspect of operational performance. There are a number ofaccessories that are typically required to be driven by the engine,comprising for example electrical generators, hydraulic pumps, fuelpumps and oil pumps. There is, in general, increasing demand forelectrical power on airframes. However there exists a problem in that,unlike engine-dedicated accessories, the power demands of an airframeare independent of the engine operating conditions, such as the spoolspeeds. For example an aircraft may require a significant level of powereven when the engine is idle or at low speeds.

Aircraft electrical power is conventionally generated by one or moreaccessory mounted generators such as an Integrated Drive Generator (IDG)and/or Variable Frequency Generator (VFG). The power to suchgenerator(s) is extracted from the high-pressure shaft since the speedvariation in that shaft is lower than that of the low-pressure shaft.IDG's feature an integral constant speed drive that ensures thegenerator operates at a fixed speed over the high-pressure range ofoperation, thereby ensuring a fixed electrical frequency output. ForVFG's control electronics are used to correct frequency variations. Thegenerators must be sized to ensure that the electrical supply meets theaircraft demands at the lowest engine speed settings.

This base level/requirement of power extraction requires that thehigh-pressure spool speed cannot drop below a lower cut-off value. Thatcut-off speed is typically higher than a desirable speed at engine idle,during taxi and/or descent during flight. Thus the high-pressure spoolis operated at higher speeds to satisfy electrical demands, therebyreducing engine efficiency and increasing thrust when it is not needed(e.g. requiring the application of aircraft brakes on the ground and/orextending the descent phase during flight).

OBJECTS AND SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a geared drive and/orcompressor arrangement which can mitigate one or more of the aboveproblems.

According to a first aspect of the invention, there is provided abooster assembly for a gas turbine engine having a first rotor assemblycomprising a low pressure turbine drivingly connected to a fan via afirst shaft and a second rotor assembly comprising a second turbinedrivingly connected to a high pressure compressor via a second shaft,the first and second rotor assemblies arranged to undergo relativerotation in use about a common axis, the booster assembly comprising afurther compressor arranged to be disposed about said common axisbetween the fan and high-pressure compressor in a direction of flow anda gearing having first and second input rotors and an output rotor, thefirst input rotor arranged to be driven by the first rotor assembly andthe second input rotor arranged to be driven by the second rotorassembly such that the output rotor drives the further compressor independence upon the difference in rotational speed between the first andsecond rotor assemblies.

The further compressor may comprise a plurality of circumferentialarrays of compressor blades in axial succession. The further compressormay comprise a compressor drum, for example comprising a series of discsattached together for co-rotation, and a casing. The casing may comprisea plurality of circumferential arrays of stator vanes depending radiallyinwardly therefrom.

The compressor may define an annular flow passage, for example between arotor drum and a casing portion thereof.

The driving of a booster in accordance with the present invention allowsthe booster to rotate at a speed greater than that of the low-pressurespool (i.e. at a speed between that of the low-pressure andhigh-pressure spools). Thus the rotational speed of the booster at idleor low engine speeds is increased above that of a conventional boosterarrangement. This can reduce the operating speed range of the boosterand improve efficiency.

The gearing may comprise an epicyclic gearing. The gearing may comprisea sun gear and a plurality of planet gears mounted on a planet gearcarrier or support.

The gearing may comprise an outer ring gear radially spaced from the sungear. The planet gears may be interposed between the sun gear and outerring.

The planet gears may be rotatably mounted on a support/carrier, forexample by bearing arrangements. The support may be rotatably mountedfor rotation about the common axis. The axes of rotation of the planetgears may be substantially parallel with, but spaced from, the commonaxis.

In one embodiment, the first input rotor of the gearing comprises a sungear and/or the second input rotor of the gearing comprises a ring gear.The output rotor may comprise a planet gear support.

In another embodiment, the first input rotor of the gearing comprises aplanet gear support and/or the second input rotor of the gearingcomprises a ring gear. The output rotor may comprise a sun gear.

In another embodiment, the first input rotor of the gearing comprises aring gear and/or the second input rotor comprises a sun gear. The outputrotor may comprise a planet gear carrier member.

The second input rotor may be integral with or else attached to thesecond shaft for co-rotation therewith. The second input rotor maycomprise an extension portion of the second shaft. The second inputrotor may be connected to the second shaft via an intermediate wall orportion which may be angled (obliquely or perpendicular) with respect tothe common axis. The second input rotor may be radially outside thesecond shaft (i.e. of larger diameter than the second shaft).

The first input rotor may be integral with or else attached to the firstshaft for co-rotation therewith. The first input rotor may be formed on,or depend from, the first shaft and may comprise an extension portionthereof. The first input rotor may comprise an annular member such as acollar or upstanding wall about the first shaft.

Either or both of the first and second input rotors may have an annulararray of gear teeth.

According to one embodiment, the output rotor of the gearing may befurther connected to, or drive, an electrical machine, such as agenerator. The output rotor may be connected to or else integral with arotor of the electrical machine. The output rotor may comprise anextension portion forming a part of the electrical machine. Theelectrical machine rotor may comprise one or more magnets.

The electrical machine may comprise a stator or else a further rotor.The stator or further rotor may comprise one or more electricalconducting members (e.g. coils) in which a current is induced by therelative rotation of the rotor. The stator or further rotor may beconcentrically arranged with the electrical machine rotor. In theexample of a further rotor, the further rotor may be driven by the firstor second rotor assembly. The further rotor may be driven by the firstshaft or an extension portion thereof.

The electrical machine rotor(s) and/or stator may be disposed about thecommon axis. Said rotor(s) may rotate about said axis.

The extraction of power using examples of the present invention, i.e. todrive a generator and/or accessory gearbox, can help to reduce engineoperability problems at low engine speeds. The invention allowselectrical power to be derived from the low-pressure spool but with aspeed range that is lower than that of the low-pressure spool.Accordingly the generator size is not as large as would typically berequired for the low-pressure spool in isolation.

According to one embodiment, the booster assembly comprises a brake forthe first rotor assembly and/or second rotor assembly. First and secondbrakes may be respectively provided.

An electrical machine may be connected to the output rotor. Theelectrical machine may serve as a starter motor upon braking of thefirst rotor assembly.

The electrical machine may serve as a generator upon release of thefirst rotor assembly, for example during normal use, or else whenbraking the second rotor assembly (e.g. during windmilling).

The invention may beneficially allow alternative starter configurationsto those posed in conventional gas turbine engine configurations.Furthermore the invention may avoid the need for long transmission drivetrains to conventionally-mounted accessory gearbox mounted generators,which can be prone to dynamic effects (such as torsional dynamiceffects) caused by accessory and/or generator loading characteristics.

The output of the gearing of the invention may drive both the further(booster) compressor and also the electrical machine.

According to a second aspect of the invention, there is provided abooster assembly for a gas turbine engine having a first rotor assemblycomprising a low pressure turbine drivingly connected to a fan via afirst shaft and a second rotor assembly comprising a second turbinedrivingly connected to a high pressure compressor via a second shaft,the first and second rotor assemblies arranged to undergo relativerotation in use about a common axis, the booster assembly comprising afurther compressor arranged to be disposed about said common axisbetween the fan and high-pressure compressor in a direction of flow anda gearing having an input rotor arranged to be driven by the first orsecond rotor assembly and at least one output rotor arranged to driveboth the further compressor and an electrical generator in use.

First and second output rotors may be provided to drive the furthercompressor and electrical generator at different speeds. Alternativelyfirst and second inputs may be provided to the gearing so as to driveboth the further compressor and the electrical generator according tothe difference in rotational speeds between the first and second rotorassemblies.

According to a third aspect of the invention, there is provided a gasturbine engine comprising the booster assembly of the first or secondaspect.

Any of the features defined above in relation to any one aspect of theinvention may be applied to any other or further aspect.

The term “co-rotation” as used herein may be considered to mean rotationin a common direction with a common speed.

BRIEF DESCRIPTION OF THE DRAWINGS

Practicable embodiments of the invention are described in further detailbelow by way of example only with reference to the accompanyingdrawings, of which:

FIG. 1 shows a half-longitudinal section through a gas turbine engineaccording to the prior art;

FIG. 2 shows a half-longitudinal section through a compressor accordingto one example of the invention;

FIG. 3 shows a half-longitudinal section through a compressor accordingto a second example of the invention;

FIG. 4 shows a half-longitudinal section through a compressor accordingto a third example of the invention; and

FIG. 5 shows a half-longitudinal section through a compressor accordingto a further example of the invention, comprising an electrical machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention derives from the premise that it is possible todrive a booster and/or generator by the difference in relative rotationbetween the high-pressure and low-pressure spools of a gas turbineengine.

Gas turbine engines using the invention may operate substantially in themanner described above in relation to FIG. 1, with the exception thatthe intermediate-pressure turbine 18 and shaft may be removed.Accordingly the intermediate-pressure compressor 14 may be replaced witha booster assembly driven by a gearing as will be described below.

Turning to FIG. 2, there is shown a booster (i.e. compressor)arrangement 24 according to one example of the invention. The compressorportion of the arrangement comprises a rotor drum 26 having a pluralityof compressor blades 28 depending radially outwardly therefrom ataxially spaced locations. The compressor blades 28 are preferablyprovided as a plurality of rows or circumferential arrays of bladesarranged about axis 11.

The blades extend into an annular compressor passageway 29. The passageis defined as the space between the rotor drum 26 (which defines aninner wall of the passage) and a concentric casing structure 32 (whichdefines an outer wall of the passage).

The compressor blades, which rotate with the drum in use, are axiallyspaced and intermediate stator vanes 30 are provided therebetween. Thestator vanes depend inwardly from the casing 32. The radially inner endsof the stator vanes may terminate at or immediately adjacent sealingformations on the rotor drum 26 to minimise any leakage fromtherebetween in use.

The passageway 29 defines a portion of the core flow passage extendingfrom a flow splitter 34 (i.e. downstream of the fan 36) to thehigh-pressure compressor 38 (only the upstream end of which is shown)and onto the combustor (not shown) in FIG. 2). The compressor is thus inthe flow path between the fan and the high-pressure compressor.

The high-pressure compressor is driven by a corresponding high-pressureshaft 40. The high pressure shaft has an extension portion 42 connectingit to an input rotor of a gearing 44. In this embodiment the extensionportion comprises a wall 46 or other support formation which isobliquely angled relative to the axis 11 and the remainder of the shaft40 so as to provide an increase in shaft diameter to accommodate thegearing 44.

The shaft extension portion terminates at a ring gear formation 48disposed about the axis 11 and having radially-inwardly facing teeth.

The gearing 44 comprises an epicyclic or planet gear arrangement inwhich a plurality of planet gears 52 are arranged between the radiallyouter ring gear 48 and a radially inner sun gear 50.

The sun gear is provided as a collar formation on the exterior surfaceof the low-pressure shaft 54 which extends between the fan 36 and thelow-pressure turbine. A portion of the low pressure shaft 54 inconcentrically inside the high-pressure shaft 40. However the lowpressure shaft 54 extends forwardly of the high pressure shaft in theaxial direction.

The low pressure shaft 54 may join with (i.e. may be integrally formedwith or attached to) a fan shaft portion 56 which drives the fan inrotation in use.

The sun gear 50 comprises a radially-outwardly facing gear teetharranged circumferentially about the shaft 54 collar. The sun gear maybe formed at a rearward portion of the fan shaft 56 or a forward portionof the low-pressure shaft 54 or at an interface or overlapping regionthere-between.

The planet gears 52 each comprise a set of teeth arranged about theircircumferential surface. The teeth thus mesh with the teeth of the ringgear 48 and the sun gear 50 concurrently.

The plurality of planet gears are typically provided as acircumferential array of gears angularly spaced (typicallyequidistantly) about the axis 11 and sun gear 50. A planet carrierstructure 58 holds the planet gears at the desired relative orientationand spacing. The planet carrier comprises one or more bearings arrangedto hold each planet gear at the required location, whilst permittingrotation of the planet gears relative thereto. In this embodiment, theplanet gears 52 each have a central hub portion which is maintained atfore and aft ends thereof within carrier structure bearings. Anintermediate wall connects the hub portion to the outer circumferentialwall on which the teeth are formed. The hub portion may be generallytubular in form.

The circumferential surface/wall of each planet gear is circular insection such that the planet gears each comprise a body of revolutionabout its own axis. The axis of rotation of each planet gear may beparallel with axis 11 but radially spaced therefrom such that each saidaxis can follow a circular path about axis 11 in use.

The planet carrier structure 58 defines a rotor, which, in thisembodiment, comprises an output rotor of the gearing 44. The planetcarrier structure is supported relative to the static engine structureby a bearing.

The planet carrier structure is drivingly connected to the compressorrotor by an interconnecting drive arm 62.

The gearing 44 is located in a cavity or enclosure surrounding thelow-pressure shaft. The gearing 44 in this embodiment is located betweenthe fan and high-pressure compressor, and preferably towards the aft ofthe booster compressor or immediately downstream thereof.

In use all of the components of the gearing, namely the gear ring 48,the sun gear 50, the planet gears 52 and the planet carrier 58 arerotatable about the common axis 11. Thus, when the gas turbine engine isin operation, the difference in rotational speed between thehigh-pressure shaft 40 and the low-pressure shaft 54 drives rotation ofthe planet gears 52 between the ring gear 48 and the sun gear 50. Theplanet gears in turn cause rotation in the planet carrier 58 about theaxis 11, which thus drives rotation of the compressor arrangement 24such that it functions as a booster.

The speed of rotation of the booster is thus adjusted in line with theinput conditions according to the low and high-pressure rotors.

The relative dimensions of the ring gear, planet gears and sun gear areselected to ensure optimal speed requirements of the booster can beachieved. Thus the booster may operate at optimal aerodynamicconditions, wherein the optimal working line is matched with that of thehigh-pressure compressor. Also the gearing can be additionally oralternatively tailored to ensure that the speed variation of the booster(i.e. the gearing output rotor) between idle and maximum speed lieswithin an acceptable range, for example for driving an electricalmachine as will be described below.

Furthermore the use of a booster configuration as shown in any of theembodiments of the invention can provide an increase in inlet pressureto the high-pressure compressor over that of a conventional two-shaftengine configuration (i.e. more akin to that of a three-shaft engineconfiguration). This enables a reduction in high-pressure compressorstages and a corresponding reduction in core engine size, therebyallowing the high-pressure spool to operate at increased rotationalspeeds.

Turning now to FIG. 3, there is shown a further embodiment which issimilar to the embodiment of FIG. 2 with the exception that the inputand output rotors are drivingly connected to different portions of theepicyclic gearbox. In the embodiment of FIG. 3, the booster compressor24 a is drivingly connected to the sun gear 50 a, for example by a shortintermediate shaft 64. The sun gear may thus be formed as a collar aboutthe intermediate shaft 64. The intermediate shaft and/or sun gear may besupported relative to the static engine structure by bearing 66.

The planet carrier 58 a in FIG. 3 depends from (i.e. is supported by)the low-pressure shaft 54 and may be connected thereto by anintermediate wall or arm formation. The ring gear arrangement 48 issubstantially unchanged and is driven by the high-pressure shaft 40.Thus the ring gear and planet bearing carrier provide input rotors,whilst the sun gear comprises an output rotor.

In FIG. 4 there is shown a further embodiment which is similar to theembodiment of FIG. 2 or 3 with the exception that the input and outputrotors are drivingly connected to different portions of the epicyclicgearbox. In the embodiment of FIG. 4, the booster compressor 24 b isdrivingly connected to the planet carrier 58 b, for example which maycomprise a short intermediate shaft portion. The booster drive arm 62 bthus connects the planet carrier to the compressor drum. The planetcarrier may be supported relative to the engine static structure by abearing arrangement.

The sun gear 50 b in this embodiment is driven by the high pressureshaft 40 and may be formed as a collar about the high pressure shaftextension portion 42 b.

The ring gear 48 b in FIG. 4 depends from (i.e. is supported by) thelow-pressure shaft 54 and may be connected thereto by an intermediatewall or arm formation 66.

In the embodiment of FIG. 4, the gearing 44 b may be shifted forwardsuch that it is in front of the booster compressor 24 b in an axialdirection. Accordingly the high-pressure shaft extension 42 b may beelongated and the ring gear may be supported by the fan shaft 56. Thegearing may be mounted within a cavity or housing surrounding thelow-pressure shaft downstream of the fan, for example within or adjacentthe front bearing housing of the engine.

By virtue of the different embodiments shown in FIGS. 2-4, it can beseen that the booster can be driven by any epicyclic gearingconfiguration in which all primary components are rotating and havingany two input rotors driving an output rotor via the planet gears. Thusany of the ring gear, planet carrier and sun gear can be drivinglyconnected to any of the high-pressure, low pressure and booster outputas required. However certain configurations are considered to providecertain benefits (i.e. offering better drive gear ratios and/or mountingconfigurations) in different engine scenarios.

In developments of the basic concept of the invention, the epicyclicarrangement can provide a speed inversion characteristic that allows thebooster to operate at relatively higher speed at lower high andlow-pressure spool speeds (i.e. at lower engine speeds) and at arelatively lower speed at higher engine speeds. In any embodiment of theinvention there may be defined a normal mode of use in which the boosteroperates at a speed between that of the low-pressure and high-pressurerotor speeds (e.g. at cruise). A second mode of use may be defined,which may comprise a reduced engine speed (e.g. an idle speed), in whichthe booster speed may be increased relative to the high-pressure spoolspeed.

Also it has been found that by using a helical tooth profile within thegearing, for certain epicyclic gearbox arrangements, the meshing teethin the gearbox the tensile axial loading between the low-pressure shaftand the booster can be at least partially counter-acted, this can reducethe effective axial loading which needs to be borne by the low-pressurespool support system, e.g. the LP location bearing.

Power can be extracted according to any of the above examples of theinvention from both the high and low-pressure spools so as to improveengine operability, especially at low engine speeds. Accordingly theinvention has been found to offer a beneficial rotor arrangement fordriving an electrical generator.

In one such embodiment a conventional accessory gearbox and/or generatorconfiguration may be driven by the gearing output/booster instead of bythe high-pressure shaft alone. In such an arrangement a radial and/orangled drive shaft (e.g. via a step-aside gearbox) may be provided aswould be understood by the person skilled in the art in order to drivean accessory gearbox mounted in the nacelle.

However in a further embodiment, an electrical machine may be integratedwith the output of the gearbox such that it is driven directly thereby.In any of the above-described embodiments, the gearbox output maycomprise a rotor of the electrical machine such that the electricalmachine is mounted for rotation about the engine axis 11, rather thanbeing offset therefrom.

An example of such an electrical machine 68 is shown in FIG. 5. Theelectrical machine in this embodiment is driven by the planet gearcarrier structure 58 c so as to induce rotation between the rotor 68 aand stator 68 b portions of the electrical machine. In such anembodiment the stator 68 b may be supported by a static portion of theengine structure.

The rotor 68 a and stator 68 b may comprise any conventional rotor orstator of an electric machine, suitable for use in a gas turbine engine,and may comprise for example one or more permanent magnets or one ormore current-carrying conductors in the form of coils. Either such rotoror stator may be arranged about engine axis 11.

The electrical machine 68, e.g. the magnet and coil assembly thereof,may be mounted at an axial location between the fan 36 and boostercompressor 24, 24 a, 24 b or HP compressor 38. The electrical machinemay be mounted radially inwardly of a bypass duct and/or core engine airintake. The electrical machine may be mounted within a cavity 70 locatedradially between the engine axis 11 and the annular passage 29 formingthe core engine air intake.

The electrical machine may be supported relative to a wall of cavity 70,for example via an internal support member 72.

In alternative embodiments of the invention for use in driving anelectrical machine, the circumferential arrangement of rotor 68 a andstator 68 b may be reversed such that the stator may be radially insideof the rotor.

The electrical machine rotor may be driven by the difference in relativerotational speeds between the high and low pressure spools of the gasturbine engine. Whilst the embodiment of FIG. 5 shows the electricalmachine rotor being driven by the planet gear carrier 58 c, it will beappreciate that the planet gear arrangement inputs and output couldalternatively be arranged, for example as indicated in FIGS. 2-4, suchthat the electrical machine could alternatively be driven by the outerring gear or inner sun gear.

The electrical machine in any embodiment can operate as an electricalgenerator during a normal mode of operation of the engine. However, theelectrical machine can also be used as a starter motor during enginestart-up, i.e. to drive the high-pressure shaft in rotation using anelectrical supply to the stator coils.

Additionally or alternatively to the generator described in relation toFIG. 5, the gearing output of the invention may be used to drive otheraccessories, such as for example a pump (e.g. an oil pump). Thus pumpsmay be at least partially driven by the fan.

The principles of the present invention may be applied within a gasturbine engine having two co-rotating or contra-rotating spools.

Whilst the present invention finds a particular application in aircraftengines, the various available gas turbine engine configurationsachievable under the present invention may be adapted to suit anyconventional gas turbine engine application, which may includeaerospace, marine, power generation amongst other propulsion orindustrial pumping applications.

What is claimed is:
 1. A booster assembly for a gas turbine engine having a first rotor assembly comprising a low pressure turbine drivingly connected to a fan via a first shaft and a second rotor assembly comprising a second turbine drivingly connected to a high pressure compressor via a second shaft, the first and second rotor assemblies arranged to undergo relative rotation in use about a common axis, the booster assembly comprising: a gearing having first and second input rotors and an output rotor, the first input rotor arranged to be driven by the first rotor assembly and the second input rotor arranged to be driven by the second rotor assembly, wherein the gearing is a mechanical epicyclic gearing comprising at least one mechanical coupling including gears and teeth, and the output rotor is drivingly connected to an electrical machine, such that the gearing drives the electrical machine.
 2. The booster assembly according to claim 1, wherein the output rotor comprises a planet gear carrier or sun gear of the epicyclic gearing.
 3. The booster assembly according to claim 1, wherein the first input rotor or second input rotor comprises a ring gear.
 4. The booster assembly according to claim 1, wherein the first or second input rotor comprises an extension portion of the respective first or second shaft.
 5. The booster assembly according to claim 1, wherein the first or second input rotor is connected to the respective first or second shaft by an intermediate wall or arm which is angled with respect to the common axis.
 6. The booster assembly according to claim 1, wherein the electrical machine is driven by the output rotor in a generator mode of operation and/or is arranged to drive the output rotor in a starter motor mode of operation.
 7. The booster assembly according to claim 1, wherein the output rotor is connected to or integral with a rotor of the electrical machine.
 8. The booster assembly according to claim 7, wherein the electrical machine rotor comprises one or more magnets.
 9. The booster assembly according to claim 1, wherein the electrical machine is disposed about the common axis.
 10. The booster assembly according to claim 1, further comprising a brake for any or any combination of the first rotor assembly, second rotor assembly, the first input rotor and/or the second input rotor.
 11. A gas turbine engine comprising: a first rotor assembly comprising a low pressure turbine drivingly connected to a fan via a first shaft; a second rotor assembly comprising a second turbine drivingly connected to a high pressure compressor via a second shaft, the first and second rotor assemblies arranged to undergo relative rotation in use about a common axis, and a booster assembly having a gearing having first and second input rotors and an output rotor, the first input rotor arranged to be driven by the first rotor assembly and the second input rotor arranged to be driven by the second rotor assembly, wherein the gearing is a mechanical epicyclic gearing comprising at least one mechanical coupling including gears and teeth, and the output rotor is drivingly connected to an electrical machine, such that the gearing drives the electrical machine. 